I agree with "GB" The way to go is with a Vacuum sealer. I put a head of iceburg in the canister, seal it and it will keep crisp and tasty for over a month.
If the head is to large for the canister I will cut it in half. After a week or so the cut edge will start turning brown. But it will still stay crisp.

I also cut up my salad vegs. Peppers, (Red, Green and yellow) onions. jacima, radishes peapods, celery, carrots and and vacuum seal them.
I leave out the cucumber and tomatos until I dish it up.
This mix will only keep about 1 week. Too, moist. Putting a paper towel in the bottom and on the top helps.